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Late Devonian--Early Mississippian Subaqueous Deltaic Facies in a Portion of the Southeastern Appalachian Basin
Richard A. Walls (1)
Evaluation of subsurface data in a portion of the southeastern Appalachian Basin permits paleoenvironmental reconstruction of a Late Devonian and/or Early Mississippian deltaic complex.
Electric logs and samples from 114 oil and gas tests were used to construct 21 stratigraphic cross sections through a six county, 2100 square mile area in southwestern Virginia and adjacent Kentucky and West Virginia.
Results obtained from petrographic analyses, grain size determinations, studies of cross sectional configurations and inferred relatinships to regional paleography indicate that there are three essentially comtemporaneous clastic facies in the study area. These are: (1) siltstone and sandy siltstone (delta front); (2) clayey siltstone (prodelta); and (3) pyritic, carbonaceous black clay shale (offshore marine). Each of the three lithofacies represents a different subaqueous environment of a north, northwest trending progradational deltaic complex.
Siltstones and shales of the study area were deposited in a shallow, euxinic sea probably no deeper than 100 feet. A low-lying drainage area, stable source, and restricted marine circulation explain the occurrence of predominantly fine-grained deposits. Abundant carbonaceous matter and pyrite imply anaerobic, reducing conditions.
Inadequate physical and paleontological control and the fact that these facies intertongue laterally raise questions concerning the validity of traditional time rock units in this area. Considering these facts, it seems unlikely that an exact Devonian-Mississippian boundary can be established in this portion of the southeastern Appalachian Basin.
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